如何在 jnr ffi 中使用带有结构的结构

Question

我有以下c代码：

#include <stdio.h>
struct Second {
    int a_number;
};
struct Top {
    struct Second second;
};
void lets_go(struct Top *top) {
    printf("The number is %dn", top->second.a_number);
}

我想从Java中做到这一点：

int main(void) {
    struct Top top = {{8}};
    lets_go(&top);
}

我也想使用 jnr-ffi，所以我查看了测试并最终得到这个：

package structs.playing;
import structs.playing.Program.Test.Top;
import structs.playing.Program.Test.Second;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;
public final class Program {
    public static interface Test {
        void lets_go(Top top);
        public static final class Second extends Struct {               
            public final Signed32 a_number = new Signed32();                
            public Second(final Runtime runtime) {
                super(runtime);
            }
        }
        public static final class Top extends Struct {              
            public Second second;                           
            public Top(final Runtime runtime) {
                super(runtime);
            }
        }
    }
    public static void main(final String[] args) {
        Test test = LibraryLoader.create(Test.class).load("test");
        Runtime runtime = Runtime.getRuntime(test);         
        Top top = new Top(runtime);
        Second second = new Second(runtime);
        top.second = second;
        second.a_number.set(7);         
        test.lets_go(top);
    }    
}

问题是根本没有设置a_number的值，所以我在输出中得到一个垃圾值，例如：

The number is 46645760

那么我如何获得与我的 C 代码相同的内容呢？

Answer 1

编辑：我花了更多的时间查看代码，我对创建结构的理解略有变化。

我相信你应该声明一个结构的所有内容并使其成为最终的，因为每次你声明一个新成员时，它都会向它所属的结构注册自己。

每个用例的结构中都有一些帮助程序函数。重载的 array（） 方法允许您注册成员或结构数组。inner（） 方法允许您注册单个结构。否则，您只需定义新的 Member 对象，它们将自动注册自己。

例如：

struct Second {
    int a_number;
};
struct Top {
    struct Second second;
    struct Second seconds[5];
    int another_number;
    int more_numbers[5];
};

表示为：

public final class Second extends Struct {
    public final Signed32 a_number = new Signed32();
    public Second(final Runtime runtime) {
        super(runtime);
    }
}
public final class Top extends Struct {              
    public final Second second = inner(new Second(getRuntime()));  
    public final Second[] seconds = array(new Second[5]);
    public final Signed32 another_number = new Signed32();
    public final Signed32[] more_numbers = array(new Signed32[5]);
    public Top(final Runtime runtime) {
        super(runtime);
    }
}

源语言：我相信正确的方法是使用接受（运行时、结构）的重载结构构造函数。https://github.com/jnr/jnr-ffi/blob/master/src/main/java/jnr/ffi/Struct.java#L129

protected Struct(Runtime runtime, Struct enclosing) {
    this(runtime);
    __info.alignment = enclosing.__info.alignment;
}

此构造函数强制封闭结构共享其内存。所以在你的例子中，我认为它看起来像这样：

package structs.playing;
import structs.playing.Program.Test.Top;
import structs.playing.Program.Test.Second;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;
public final class Program {
    public static interface Test {
        void lets_go(Top top);
        public static final class Second extends Struct {               
            public final Signed32 a_number = new Signed32();                
            public Second(final Runtime runtime, final Struct enclosing) {
                super(runtime, enclosing);
            }
        }
        public static final class Top extends Struct {              
            public Second second;                           
            public Top(final Runtime runtime) {
                super(runtime);
            }
        }
    }
    public static void main(final String[] args) {
        Test test = LibraryLoader.create(Test.class).load("test");
        Runtime runtime = Runtime.getRuntime(test);         
        Top top = new Top(runtime);
        Second second = new Second(runtime, top);
        top.second = second;
        second.a_number.set(7);         
        test.lets_go(top);
    }    
}

请注意对 Second 构造函数的更改，并注意我将 Top 对象传递给 Second 对象，以便它知道 top 正在包围它。这没有经过测试，只是分享我在尝试理解代码时发现的内容。

我认为在您的示例中发生的事情是第二个对象正在分配自己的内存，而 Top 对此一无所知。

如果这不起作用，我建议考虑做这样的事情：

public static final class Top extends Struct {              
    public Second second = new Second(getRuntime(), this);                           
    public Top(final Runtime runtime) {
        super(runtime);
    }
}

Answer 2

分配结构时，如行所示

top.second = second;

在你的Java代码中，结构从second复制到top.second，以便它们成为不同内存区域中的独立实体。稍后，当您将 7 分配给以下行中 second 的 a_number 属性时：

second.a_number.set(7);

top.second 的相应属性保持不变，因为它们不是同一个对象。

为了最终得到与 C 代码相同的结果，请尝试将 main 方法更改为：

public static void main(final String[] args) {
    Test test = LibraryLoader.create(Test.class).load("test");
    Runtime runtime = Runtime.getRuntime(test);         
    Top top = new Top(runtime);
    top.second.a_number.set(8);         
    test.lets_go(top);
}

初始化新的Second对象不是必需的，因为在初始化Top对象的过程中，内存已分配给top.second。

Answer 3

我已经

想通了（顺便说一下，我知道成员应该是私有的并且包装在属性中，但我想使代码片段尽可能小，这不是生产质量的代码）...

如果你把一个指针成员变量放到结构中，你可以在构造从属结构体时使用它的内存，就像这样......

package structs.playing;
import structs.playing.Program.Test.Top;
import jnr.ffi.LibraryLoader;
import jnr.ffi.Runtime;
import jnr.ffi.Struct;
public final class Program {
    public static interface Test {
        void lets_go(Top top);
        public static final class Second extends Struct {
            public final Signed32 a_number = new Signed32();
            public Second(final Runtime runtime) {
                super(runtime);
            }           
        }
        public static final class Top extends Struct {
            private final Pointer secondPointer = new Pointer();            
            public final Second second;
            public Top(final Runtime runtime) {
                super(runtime);                             
                second = new Second(runtime); 
                second.useMemory(secondPointer.getMemory());
            }           
        }
    }
    public static void main(final String[] args) {
         Test test = LibraryLoader.create(Test.class).load("test");
         Runtime runtime = Runtime.getRuntime(test);         
         Top top = new Top(runtime);
         top.second.a_number.set(8);         
         test.lets_go(top);
    }
}